1. Field of the Invention
The present invention relates to methods of forming a semiconductor device and, more particularly, to methods of forming a semiconductor device including a phase change material layer.
2. Description of the Related Art
Phase change memory devices are non-volatile memory devices that employ a phase change material as a data storage element. The phase change material has two stable states which exhibit different resistive characteristics, for example either an amorphous state or a crystalline state. If the phase change material is heated to a temperature higher than the melting point thereof and quenched down rapidly, the phase change material may become the amorphous state. In contrast, if the phase change material is heated to a temperature within the range of the melting point to a crystallization temperature thereof and cooled down slowly, the phase change material may become the crystalline state. Accordingly, the phase change material may be used as a data storage element to program a logic data “0” or a logic data “1”, and the data stored in the phase change material can be read out by applying a predetermined voltage to the phase change material and sensing a current that passing therethrough.
Joule heat is used to change the crystalline structure of the phase change material. The joule heat may be generated by forcing a heating current into a conductive heater which is connected to the phase change material, and the resultant joule heat is thermally conducted to the phase change material. The temperature of the phase change material depends on the amount of the heating current. In general, at least several milli-amperes may be required to change the crystalline structure of the phase change material. Thus, there may be some limitations in improving the integration density of the phase change memory device and in reducing the power consumption thereof.
As the semiconductor devices such as phase change memory devices become more highly integrated, methods for reducing heating current are continuously explored. Recently, a method of forming the phase change material in a confined space (for example, a hole, etc.) has been proposed in order to reduce the heating current. If the phase change material is formed in the confined space, a contact area between the phase change material and the conductive heater can be decreased to reduce the heating current that is required to change the crystalline structure of the phase change material.
The phase change material may be deposited using a sputtering technique. In this case, the phase change material may not completely fill the confined space such as the hole since the sputtering technique exhibits poor step coverage. Thus, a chemical vapor deposition (CVD) technique has recently been proposed to form the phase change material layer such as a GST (Ge—Sb—Te) layer. However, in the event that the phase change material layer is formed using the CVD technique that employs source gases which are well known in the art, some problems may occur. For example, when the GST layer is formed using the CVD technique, a GeH4 gas or a Ge(allyl)4 gas may be widely used as the source gas and the GeH4 gas or the Ge(allyl)4 gas is decomposed at a temperature of about 400° C. Accordingly, the CVD process using the GeH4 gas or the Ge(allyl)4 gas as a source gas should be performed at a temperature of 400° C. or the higher. However, the GST layer may begin to volatilize at a temperature of about 350° C. Thus, the GST layer may volatilize during deposition of the GST layer using the conventional CVD technique that employs the GeH4 gas or the Ge(allyl)4 gas as a source gas. As a result, the conventional CVD GST layer may be discontinuously and/or non-uniformly formed to degrade characteristics of the phase change memory device.